MPO and anti-neutrophil cytoplasmic antibody-associated vasculitis: antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) are a group of small vessel vasculitis that is associated with the formation of autoantibodies against cytoplasmic components of neutrophils, such as proteinase 3 (PR3)-ANCA and myeloperoxidase (MPO)-ANCA.